The present invention concerns certain special purpose lipid vesicles, specifically lipid vesicles with voids and oxygen carrying capacity. These special lipid vesicles have significant uses industrially. The vesicles with voids can be used as whiteners for the paint industry, while oxygen carrying vesicles are particularly useful in the biological sciences, e.g., as artificial oxygen suppliers. While the different aspects of the invention may appear somewhat diverse, the broad applicability of the methods described herein to both aspects of the invention shall be readily apparent from the following description.
Paucilamellar lipid vesicles, e.g., those described in U.S. Pat. No. 4,911,928, the disclosure of which is incorporated herein by reference, have 2-10 substantially spherical lipid bilayers surrounding a large, amorphous cavity or core. The lipid bilayers are hydrated, having bound water entrapped between the bilayers. The central cavity may encapsulate an aqueous solution, or a water immiscible oil or wax can fill part or all of the core.
There have been numerous attempts at drying aqueous filled lipid vesicles, primarily using a variety of lyophilization methods. In most instances, a cryoprotectant such as high molecular weight dextran or other carbohydrate has been used to preserve the lipid wall structure as the aqueous solution is removed. Without this cryoprotectant, the dehydrated bilayers collapse, forming a solid bilamellar array which is often distinguishable microscopically by visible optical bifringence. These lyophilization methods have been moderately successful, allowing rehydration (and, accordingly, reconstitution) of the lipid vesicles. The cryoprotectants are deposited around the bilayers, allowing reformation of the vesicles from the lamellar skeletons upon rehydration. However, unless rehydration is by using a vapor rather than liquid phase, there normally is some loss of encapsulated water soluble material into the external aqueous phase. The use of cryoprotectants provides a measure of stability to the spherical bilayer shells by preventing the aggregation of vesicles during dehydration. This does not prevent collapse of the vesicle core for paucilamellar lipid vesicles.
The situation is somewhat different if a nonvolatile hydrophobic material such as a paraffin wax is encapsulated in the amorphous central cavity of a paucilamellar vesicle. These vesicles have a monolayer of a surfactant surrounding the hydrophobic core, either from cannibalizing the wall material or by adding a separate, indifferent (or nonvesicle forming surfactant) in the manufacture of the vesicle. As the water is removed, the surfactant stabilized wax droplets limit the collapse of this central core, forming tiny, solid particles and leading to relatively easy rehydration.
While it is possible to make vesicles with a hydrophobic core encapsulating a liquid which is more volatile than the aqueous solution bound in the bilayers, this procedure does not lead to dry vesicles with a void in the center. These volatile materials, e.g., organic solvents, volatilize before the water as the vesicles are dried. As the drying continues, the core which had been filled by the volatile material leaches water from the bilayers into the central cavity. The vesicles shrink, forming small aqueous filled paucilamellar lipid vesicles that give up their water as if there had never been any volatile liquid in the core.
None of the described methods have provided means for forming stable, dry lipid vesicles with a void-like gaseous (or air) filled core. This particular type of vesicle would have particular applicability in the paint and cosmetic industries. The two types of whiteners which presently provide the primary whitening capacity for the paint and cosmetic industries are titanium dioxide pigments (TiO.sub.2) and polymer particles with voids. Although TiO.sub.2 has a long history as a whitener, the paint and cosmetic industries have been trying to move away from this type of heavy metal whitener. Rohm & Haas developed a whitener primarily for the paint industry, sold under the trademark Rhopaque.TM., which is a small polymer sphere having an air space of less than 0.2 mm. This small but distinct central cavity provides optical scattering so the particle acts as a whitener. A lipid vesicle which could be dried and have a similar central cavity could provide the same whitening capacity while having the further advantages of biodegradability and cost competitiveness.
Since lipid vesicles having this type of gas-filled center cannot be made from aqueous filled vesicles, nonvolatile hydrophobic filled vesicles, or vesicles filled with highly volatile hydrophobic materials, the present invention relates to the use of moderately volatile liquids as an attempt to solve the problem. As used herein, the term "moderately volatile liquid" means and implies a liquid that is less volatile than water but more volatile than the lipids which form the bilayer structure of the vesicles. These moderately volatile liquids have a vapor pressure such that they are able to leach out of the lipid vesicles when air dried, dried under a vacuum, or lyophilized, but they do so at a slower rate than the encapsulated aqueous solution. Accordingly, substantially all of the aqueous solution encapsulated in the vesicle, as well as the aqueous solution surrounding the vesicle, evaporates before the moderately volatile liquid and the moderately volatile liquid provides structure to the vesicle as the water is removed.
Other requirements for this moderately volatile liquid included substantial immiscibility in aqueous solutions and the lipids forming the vesicles, as well as substantial unreactivity with the lipid. A class of materials which meet all these requirements are the perfluorocarbons. Perfluorocarbons are, primarily, alkanes or cycloalkanes having all of the hydrogens replaced with fluorines. Certain of these perfluorocarbons, primarily, perfluorodecalin and perfluorotripropylamine, have an additional advantage; they can function as oxygen carriers. Most perfluorocarbons have a density of about two, have a high air-and oxygen-carrying capacity, and are substantially unreactive with most materials. Encapsulation of oxygen-rich perfluorocarbons could have substantial value for use in cosmetics or in fermentation because of their high in vitro oxygenation capacity. In addition, these materials have promise for a substantial number of medical uses. For example, for these materials would be advantageous if used as an infusion after trauma, for infusion for radiotherapy, for wound perfusion, in angioplasty, or in radioimaging.
Accordingly, an object of the invention is to provide a method of encapsulating perfluorocarbons and other moderately volatile liquids.
Another object of the invention is to provide a method of forming gas-filled lipid vesicles.
A further object of the invention is to provide a whitener which could be used in a variety of industrial applications.
These and other objects and features of the invention will be apparent from the following description.